The invention relates to a method of protecting the electric drive of a vehicle, in particular an electric automobile, against overload when the nominal (i.e. the "rated") power is temporarily exceeded.
An advantage of electric vehicles is that they can be overloaded for short periods or, in other words, their nominal (i.e. "rated") power (maximum permissible contnuous power) can be exceeded for a short period. During the operation of electric vehicles, this advantage is used for starting and accelerating and for driving up slopes and ramps. However, the operation at overload must be limited timewise in order to protect the electric drive from damage.
One overload protection is provided by the thermal monitoring of all of the drive components. When specified temperature thresholds are exceeded, the power of the drive is automatically reduced. The reduction in power leads to a reduction in the traveling speed. The speed reduction occurs as a surprise to the vehicle operator, which can lead to dangerous traffic situations.
Another possiblity for overload protection is to monitor operating current integrated over time. As an example, a circuit configuration is known from German Patent DE-PS 24 31 540 for protecting an electrical machine against excessive heating. In that configuration, the rotor temperature is influenced by limiting the motor current in such a way that the maximum permissible temperature is not exceeded. For this purpose, the square of the measured current is integrated and the integral value is compared with a temperature value corresponding to the nominal power. The motor current is reduced in the case of excessive integral values. If this control is used for electric vehicles, surprising effects can again occur for the vehicle operator because the maximum speed of the vehicle depends on the previous history of the driving power. It is not possible to select a predictable and reproducible driving power (maximum speed).
It is accordingly an object of the invention to provide a method of protecting the electric drive of a vehicle against overload, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods of this general type, which makes it possible to keep within driving powers which can be reproduced and which avoids the occurrence of surprising events for the vehicle operator.